Effect of reversible inactivation of macaque lateral intraparietal area onvisual and memory saccades

Previous studies from our laboratory identified a parietal eye held in the primate lateral intraparietal sulcus, the lateral intraparietal area (area LTP). Here we further explore the role of area LIP in processing saccadic e ye movements by observing the effects of reversible inactivation of this ar ea. One to 2 mu l of muscimol (8 mg/ml) were injected at locations where sa ccade-related activities were recorded for each lesion experiment. After th e muscimol injection we observed in two macaque monkeys consistent effects on both the metrics and dynamics of saccadic eye movements at many injectio n sites. These effects usually took place within 10-30 min and disappeared after 5-6 h in most cases and certainly when tested the next day. After mus cimol injection memory saccades directed toward the contralesional and uppe r space became hypometric, and in one monkey those to the ipsilesional spac e were slightly but significantly hypermetric. In some cases, the scatter o f the end points of memory saccades was also increased. On the other hand, the metrics of visual saccades remained relatively intact. Latency for both visual and memory saccades toward the contralesional space was increased a nd in many cases displayed a higher variance after muscimol lesion. At many injection sites we also observed an increase of latency for visual and mem ory saccades toward the upper space. The peak velocities for memory saccade s toward the contralesional space were decreased after muscimol injection. The peak velocities of visual saccades were not significantly different fro m those of the controls. The duration of saccadic eye movements either to t he ipsilesional or contralesional space remained relatively the same for bo th Visual and memory saccades. Overall these results demonstrated that we w ere able to selectively inactivate area LIP and observe effects on saccadic eye movements. Together with our previous recording studies these results further support the view that area LIP plays a direct role in processing in coming sensory information to program saccadic eye movements. The results a re consistent with our unit recording data and microstimulation studies, wh ich suggest that area LIP represents contralateral space and also has a bia s for the upper visual field.